Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) technology is a mobile broadband wireless communication technology in which transmissions from base stations (e.g., evolved Node Bs (eNBs)) to mobile stations (e.g., User Equipments (UEs)) are sent using Orthogonal Frequency Division Multiplexing (OFDM). OFDM splits a signal into multiple parallel sub-carriers in frequency. The basic unit of transmission in LTE is a Resource Block (RB), which, in its most common configuration, includes twelve (12) subcarriers and seven (7) OFDM symbols (which may provide one (1) slot). A unit of one (1) subcarrier and one (1) OFDM symbol is referred to as a Resource Element (RE), which is illustrated in FIG. 1A. Thus, an RB may include eighty-four (84) REs. Referring to FIG. 1B, an LTE radio subframe includes multiple RBs in frequency, with the number of RBs determining the bandwidth of the system and two (2) slots in time. Furthermore, the two (2) RBs in a subframe that are adjacent in time are denoted as an RB pair.
In the time domain, LTE downlink transmissions are organized into radio frames of ten (10) milliseconds (ms), each radio frame consisting of ten (10) equally-sized subframes of length Tsubframe=one (1) ms. The signal transmitted by the eNB in a downlink (the link carrying transmissions from the eNB to the UE) subframe may be transmitted from multiple antennas and the signal may be received at a UE that has multiple antennas. The radio channel distorts the transmitted signals from the multiple antenna ports. To demodulate any transmissions on the downlink, a UE relies on Reference Symbols (RS) that are transmitted on the downlink. These RSs and their position in the time-frequency grid are known to the UE and hence can be used to determine channel estimates by measuring the effect of the radio channel on these symbols.
Messages transmitted over the radio link to users can be broadly classified as control messages or data messages. Control messages are used to facilitate the proper operation of the system, as well as proper operation of each UE within the system. Control messages could include commands to control functions such as the transmitted power from a UE, signaling of RBs within which the data is to be received by the UE or transmitted from the UE, and so on.
Examples of control messages are the Physical Downlink Control Channel (PDCCH), which, for example, carries scheduling information and power control messages; the physical HARQ indicator channel (PHICH), which carries ACK/NACK in response to a previous uplink transmission; and the Physical Broadcast Channel (PBCH), which carries system information. Also, primary and secondary synchronization signals can be seen as control signals (PSS/SSS) with fixed locations and periodicity in time and frequency so that UEs that initially access the network can find them and synchronize.
The PBCH is not scheduled by a PDCCH transmission but has a fixed location relative to the primary and secondary synchronization signals (PSS/SSS). Therefore, the UE can receive the system information transmitted in BCH before it is able to read the PDCCH.
In LTE Release 10 (Rel.10 or Rel-10), all control messages to UEs are demodulated using the Common Reference Signals (CRS). Hence, they have a cell wide coverage to reach all UEs in the cell without having knowledge about their position. An exception is the PSS and SSS, which are stand-alone and do not need reception of CRS before demodulation. The first one to four (1-4) OFDM symbols, depending on the configuration, in a subframe are reserved to contain such control information, as illustrated in FIGS. 1B and 1C. Control messages could be categorized into those types of messages that need to be sent only to one (1) UE (i.e., UE-specific control) and those that need to be sent to all UEs or some subset of UEs numbering more than one (i.e., common control) within the cell being covered by the eNB.
Control messages of PDCCH type are demodulated using CRS and transmitted in multiples of units called Control Channel Elements (CCEs), where each CCE contains thirty-six (36) REs. A PDCCH may have Aggregation Level (AL) of 1, 2, 4 or 8 CCEs to allow for link adaptation of the control message. Furthermore, each CCE is mapped to nine (9) Resource Element Groups (REG) consisting of four (4) RE each. These REGs are distributed over the whole system bandwidth to provide frequency diversity for a CCE. Hence, a PDCCH, which consists of up to eight (8) CCEs, may span the entire system bandwidth in the first one to four (1-4) OFDM symbols, depending on the configuration. For example, FIG. 1C illustrates the mapping of one (1) CCE belonging to a PDCCH to a control region that spans a whole system bandwidth.
After channel coding, scrambling, modulation and interleaving of the control information, the modulated symbols are mapped to the resource elements in the control region. As described herein, CCEs have been defined, where each CCE maps to thirty-six (36) REs. By choosing the aggregation level, link-adaptation of the PDCCH obtained. In total, there are NCCE CCEs available for all the PDCCH to be transmitted in the subframe, and the number NCCE varies from subframe to subframe, depending on the number of control symbols n and the number of configured PHICH resources.
As NCCE varies from subframe to subframe, the terminal would need to blindly determine the position as well as the number of CCEs used for its PDCCH, which can be a computationally intensive decoding task. Therefore, some restrictions in the number of possible blind decodings a terminal needs to go through have been introduced in LTE Release 8 (Rel.8 or Rel-8). For instance, the CCEs are numbered and CCE aggregation levels of size K can only start on CCE numbers evenly divisible by K, as illustrated in FIG. 1D. As an example, FIG. 1D illustrates CCE aggregation showing Aggregation Levels (ALs) 8, 4, 2, and 1.
The set of CCE where a terminal (e.g., a UE) needs to blindly decode and search for a valid PDCCH is called the UE's search space. This is the set of CCEs on an AL a terminal should monitor for scheduling assignments or other control information. In each subframe and on each AL, a terminal will attempt to decode all the PDCCHs that can be formed from the CCEs in its search space. If a Cyclic Redundancy Check (CRC) checks, then the content of the PDCCH is assumed to be valid for the terminal and it further processes the received information. Often, two (2) or more terminals will have overlapping search spaces, and the network has to select one (1) of them for scheduling of the control channel. When this happens, the non-scheduled terminal is said to be blocked. The search spaces for a UE vary pseudo-randomly from subframe to subframe to reduce/minimize this blocking probability. Moreover, FIG. 1E is a flowchart illustrating processing steps of all the PDCCHs to be transmitted in a subframe. For example, FIG. 1E illustrates that a PDCCH message may be structured into CCEs (Block 101). Control information may then be scrambled and modulated (Block 102). Layer mapping and/or transmit diversity operations may optionally be performed (Block 103). Quadruplex-based interleaving may be performed (Block 104). A cyclic shift may be performed based on a cell identification (Block 105). Moreover, mapping to REGs may be performed (Block 106).
In LTE Release 11 (Rel.11 or Rel-11), it has been agreed to introduce UE-specific transmission for control information in the form of enhanced control channels by allowing the transmission of generic control messages to a UE using such transmissions based on UE-specific reference signals and by placement in the data region, as illustrated in FIG. 1F. This is commonly known as the enhanced PDCCH (ePDCCH), enhanced PHICH (ePHICH), and so on. For the enhanced control channel in Rel.11, it has been agreed to use antenna ports pε{107, 108, 109, 110} for demodulation, i.e., the same antenna ports that are used for Physical Downlink Shared Channel (PDSCH) transmission using UE-specific Reference Symbols (RSs). This enhancement means that precoding gains can be achieved also for the control channels. Another benefit is that different PRB pairs (or enhanced control regions, see FIG. 1I) can be allocated to different cells or different transmission points within a cell, and thereby inter-cell or inter-point interference coordination between control channels can be achieved. This is especially useful for an HetNet scenario.
FIG. 1F illustrates a downlink subframe showing ten (10) Resource Block (RB) pairs and a configuration of three (3) ePDCCH regions (111, 112, 113) having a size of one (1) Physical Resource Block (PRB) pair each. The remaining RB pairs can be used for Physical Downlink Shared Channel (PDSCH) transmissions.
The same enhanced control region (see FIG. 1I) can be used in different transmission points within a cell or belong to different cells, that are not highly interfering with each other. A typical case is the shared-cell scenario, where a macro cell contains lower-power pico nodes within its coverage area, having (or being associated with) the same synchronization signal/cell ID.
For example, FIG. 1G illustrates a heterogeneous network scenario, where the dashed line in FIG. 1G indicates a macro cell coverage area 120, and 121, 122, and 123 correspond to the coverage areas of three (3) pico nodes 131, 132, and 133, respectively. In a shared-cell scenario, pico node coverage areas 121, 122, and 123 and macrocell coverage area 120 have the same cell ID, e.g., the same synchronization signal (i.e., transmitted or being associated with the same synchronization signal).
In pico nodes that are geographically separated, such as pico nodes 132 and 133 in FIG. 1G, the same enhanced control region, i.e., the same PRBs used for the ePDCCH, can be re-used. In this manner, the total control channel capacity in the shared cell will increase because a given PRB resource is re-used, potentially multiple times, in different parts of the cell. This ensures that area splitting gains are obtained. For example, in FIG. 1H, pico nodes 132 and 133 share the enhanced control region, whereas pico node 131, due to the proximity to pico node 132, is at risk of interfering and is therefore assigned an enhanced control region that is non-overlapping. Interference coordination between pico nodes 131 and 132, or, equivalently, transmission points 131 and 132, within a shared cell is thereby achieved. In some cases, a UE may need to receive part of the control channel signaling from the macro cell and the other part of the control signaling from the nearby pico cell.
This area splitting and control channel frequency coordination is not possible with the PDCCH because the PDCCH spans the whole bandwidth. The PDCCH does not provide possibility to use UE-specific precoding because it relies on the use of CRS for demodulation.
FIG. 1H shows an ePDCCH, which, similar to the CCE in the PDCCH, is divided into multiple groups and mapped to one of the enhanced control regions 111 and 112. In FIG. 1H, the enhanced control region does not start at OFDM symbol zero, to accommodate simultaneous transmission of a PDCCH in the subframe. However, as described herein, there may be carrier types in future LTE releases that do not have a PDCCH, in which case the enhanced control region could start from OFDM symbol zero within the subframe.
FIG. 1I illustrates a downlink subframe showing a CCE belonging to an ePDCCH mapped to one of the enhanced control regions, to achieve localized transmission. Moreover, FIG. 1J illustrates a downlink subframe showing a CCE belonging to an ePDCCH mapped to multiple ones of the enhanced control regions, to achieve distributed transmission and frequency diversity or subband precoding.
Even if the enhanced control channel enables UE-specific precoding and such localized transmission as illustrated in FIG. 1I, it can, in some cases, be useful to be able to transmit an enhanced control channel in a broadcast, wide area coverage fashion. This is useful if the eNB does not have reliable information to perform precoding towards a certain UE, and then a wide area coverage transmission may be more robust.
Another case is when the particular control message is intended for more than one UE, and UE-specific precoding cannot be used. An example is the transmission of the common control information using PDCCH (i.e., in the common search space (CSS)).
In any of these cases, a distributed transmission over enhanced control regions can be used. For example, FIG. 1J illustrates that four (4) parts 141-444 belonging to the same ePDCCH are distributed over the enhanced control regions.
It has been agreed in the 3GPP ePDCCH development that both distributed and localized transmission of an ePDCCH should be supported corresponding to FIGS. 1J and 1I, respectively.
When distributed transmission is used, then it may also be beneficial if antenna diversity can be achieved to increase/maximize the diversity order of an ePDCCH message. On the other hand, sometimes only wideband channel quality and wideband precoding information is available at the eNB for which it could be useful to perform a distributed transmission but with UE-specific, wideband, precoding.
To accommodate distributed transmission of enhanced control channels as well as supporting multiple options for localized transmission, a set of PRB pairs distributed in frequency may have to be allocated for the enhanced control region. To support higher control channel capacity than provided by a single set of PRB pairs, multiple sets may be allocated for the enhanced control region. This allocation may be done UE-specifically. In other words, different allocations for different UEs may be allocated simultaneously. Because PRB pairs used for PDSCH transmission often are allocated in terms of Resource Block Groups (RBGs) (i.e., groups of PRB pairs contiguous in frequency), it may be beneficial to limit the number of RBGs containing the enhanced control region for a given capacity. This is achieved by allocating multiple sets of PRB pairs from the same RBG. The group of RBGs forming the multiple of sets is referred to as a cluster. Even though a PRB pair is part of the enhanced control region, it may be used for PDSCH if no other transmissions take place. The described division of resources is illustrated in FIG. 1K.
For example, FIG. 1K illustrates a definition of sets and clusters, where the number of sets equals the Resource Block Group (RBG) size. The number of RBGs per cluster is in this example set to four (4), which corresponds to four (4) PRB pairs per set. A distributed ePDCCH transmission is mapped within one (1) set. If additional control resources are needed, then additional clusters can be configured.
Accordingly, in LTE, an enhanced control channel referred to as the ePDCCH is being defined. A UE uses blind decoding techniques to receive ePDCCH messages with several blind decoding candidates being tested. In Rel-8, the assignment of a number of blind decoding candidates for each aggregation level of the PDCCH is known to the UE. For the ePDCCH, the entire space over which an ePDCCH may be received is partitioned into sets. Making this partition known to UEs via Radio Resource Control (RRC) signaling can lead to significant extra overhead. Moreover, specification of these values as is done in Rel-8 is not simple, due to the number of combinations of sets of potentially different sizes that may be configured for a UE.
Furthermore, frequency multiplexing gains sometimes suffer when blind decoding candidates tested by a UE occur consecutively. Also, when two sets configured for a UE are fully or partially overlapping, the currently-used hash function in Rel-8 does not allow different blind decoding candidates for these sets.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.